Peter Lynch wrote:
"You say it's been some years since the locks received any TLC.
Have you tried oiling the lock mechanism and/or hooks since this
problem occured?"

I didn't actually say that but I may have given that impression, sorry.
I only recently lubricated the lock itself with graphite powder, and
the moving parts all move freely when the door is open. The problem
seems to be that I've inadvertently messed up the alignment so that
when the door is shut, the catches are probably not engaging properly
with the frame any more. I suspect the hinge adjustment is what I've
messed up, as suggested by Terry.

I'll have a go at adjusting the hinges tonight to see if I can sort it
out. Thanks for everyone's advice so far.

Cheers

Flummoxed